Sintered contact material, method for preparing it, and corresponding contact facings

ABSTRACT

A sintered contact material comprising silver and nickel is characterized according to the invention in that the mass fraction of nickel is between 5 and 50%, and in that the nickel is present in the silver microstructure with average particle sizes (d) 1 μm&lt;d&lt;10 μm in largely homogeneous dispersion. A suitable method for preparing said sintered contact material is characterized in that, prior to sintering the nickel is introduced, in the way of mechanical alloying, into the silver microstructure, this operation taking place under an air atmosphere. Contact facings manufactured therefrom can be formed as strips or sections by means of extrusion, as individual contact pieces by means of a shaped part technique, and in each case as a two-layer structure.

BACKGROUND OF THE INVENTION

The invention relates to a sintered contact material comprising silverand nickel, to a method for preparing it, and to contact facings madetherefrom.

Good utility for switching currents in switchgear of power engineeringhas been shown in the past by contact materials comprising silver (Ag)and nickel (Ni). The preparation of such contact materials and themanufacture and testing of corresponding contact pieces is described indetail in Int. J. Powder Metallurgy and Powder Technology, Vol. 12(1976), p. 219-228.

To prepare a contact material comprising silver and nickel, according tothe prior art silver powder and nickel powder are customarily wet-mixedin a mixer, dried, pressure-moulded and sintered under a reducingatmosphere. The fineness of the microstructure essentially depends onthe size of the starting powders used. Such relationships are describedin detail in the monograph by H. Schreiner "Pulvermetallurgieelektrischer Kontakte", Powder metallurgy of electrical contacts!,Springer-Verlag (1976), pages 105 to 140. In particular, an AgNimaterial prepared by means of precipitated powder and having averagegrain sizes of 1 μm is specified.

It had previously been assumed that, in the case of contact materialscomprising silver and nickel, the nickel particles must be present inthe silver in as small and finely dispersed form as possible, in orderfor the contact to have good switching characteristics. A suitable wayof achieving this, in principle, is the known method of mechanicalalloying. As early a publication as JP-A 66/33090 discloses a method forpreparing materials for electrical contacts on a silver basis, a furthercomponent being chosen in the form of a metal which is insoluble or onlyslightly soluble in silver.

This metal, in particular, is nickel, iron, tungsten or another metalwhich does not form a mixed crystal with silver or for which, onthermodynamical grounds, according to the state diagram there is thetendency towards segregation.

JP-A 66/33090 aims for a mixed crystal-like constitution of thematerial. To this end, electrolyte/silver powder and carbonyl-nickelpowder are mixed in a ball mill with steel balls under so-called styrenegas for extended periods, for example up to 300 h, in order to obtain amechanically alloyed powder. The aim is for the powder thus obtained tohave grain sizes below 0.01 μm. In an X-ray diffraction analysis, thedisappearance of nickel reflections and thus the presence of anamorphous alloy was confirmed in this instance. When contacts arefabricated from an alloy powder thus prepared involving alternatesintering and pressing steps it should be possible for secondarysegregations to be formed, but with the grain size of the nickelparticles limited to 1 μm.

It was found that when mechanically alloyed silver-nickel powders havingthe above-described amorphous character are used, undesirable sideeffects may occur which result in comparatively poor contactcharacteristics.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an appropriate remedy. Acontact material comprising silver and nickel is to be provided which,compared with conventional silver-nickel materials, has improved contactproperties. At the same time, the appropriate preparation method andcorresponding contact facings are to be described.

The object is achieved, according to the invention, in the case of asintered contact material comprising silver and nickel, by the massfraction of nickel being between 5 and 50%, and by the nickel beingpresent in the silver microstructure with average particle sizes 1μm<d<10 μm in largely homogeneous dispersion.

Preferably, the average particle size of the nickel is a d<5 μm,especially d<3 μm. For the particle size distributions specified, theaverage distance D of the nickel particles should be between 5 and 10μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of the material AgNi10 with a detailed viewshowing the average distance D between two nickel particles forparticles having a particle size d of about 3 μm.

FIG. 2 is a photomicrograph of the material AgNi40.

DETAILED DESCRIPTION OF THE INVENTION

The method for preparing the specified sintered contact materialcomprising silver and nickel is characterized, according to theinvention, in that prior to sintering, the nickel is introduced, in theway of mechanical alloying, into the silver microstructure, thisoperation taking place under an atmosphere of air. The startingmaterials used in the process are either silver powder and nickel powderor alternatively a granular material comprising silver and nickel.Preferably, particle size distributions below 500 μm, preferably below100 μm, especially below 50 μm are possible. Mixing in the way ofmechanical alloying takes place in a ball mill and continues until alamella microstructure has formed, with Ni lamella widths which are verymuch smaller than the particle diameter of the starting powder. Such adegree of refinement of the microstructure falls within the range of thedetection limit of an optical microscope.

The invention makes it possible, employing the silver-nickel powderprepared in the way of mechanical alloying, to employ pressure-mouldingsuch as extrusion or a shaped part technique and sintering under areducing atmosphere, for contact facings to be fabricated. Preferably,the contact facings are fashioned as strips or sections or as contactpieces and are used in a power engineering switching device.

In contrast to the prior art, the mechanical alloying in the case of theinvention is not carried out under a protective gas. Instead, normalatmospheric air is employed. Nor is the mixing, as particularly in JP-A66/33090, carried out for as long as possible in order to obtain as fineas possible an alloyed powder. Instead deliberate advantage is taken ofthe operation of mechanical alloying being carried out under air. As aresult, oxide skins are formed on the particles which have the sameeffect as fusion-inhibiting additives. The oxides on the surface of theparticles further contribute to embrittlement of the composite particlesand thus to more rapid refinement of the microstructure. Compared withmechanial alloying under inert gas, the mechanical alloying operation isconsiderably shortened.

Further details and advantages of the invention can be gathered from thefollowing description of working examples, reference being made tomicrographs with accompanying enlarged detail and a table with theresults of an electrical test. Shown in 400 fold magnification are inFIG. 1 the micrograph of a material AgNi10 and in FIG. 2 the micrographof a material AgNi40.

To prepare the materials AgNi10 and AgNi40, silver powders having aparticle size distribution <300 μm and nickel powders having a particlesize distribution <150 μm are used as starting materials. After havingbeen weighed in accordingly, the powders are placed into a ball mill(Attritor) and there alloyed mechanically until the nickel in themicrostructure being formed has a size of <3 μm and is presenthomogeneously in the silver. Preparation takes place in the ball mill inan atmosphere of air and without waxes as further additives.

The microstructure refinement produced during mechanical alloying isaccompanied by a change in the particle shape and particle size of thepowder. Processing under an atmosphere of air deliberately incurs theformation of oxide skins on the particles.

After mixing in the way of mechanical alloying, contact facings areproduced in a known manner by pressure-moulding and sintering under areducing atmosphere. Possible methods of pressure moulding are eitherextrusion to fabricate strips or sections, or else the so-called shapedpart technique for fabricating individual contact pieces. At the sametime it is advantageous to produce two-layer contact facings ortwo-layer contact pieces comprising a first layer of silver-nickel and asecond layer of pure silver, in order to ensure a reliable bondingtechnique to the contact carrier.

The micrographs according to FIG. 1 and FIG. 2 show the material AgNi10on the one hand and AgNi40 on the other hand. This demonstrates thehomogeneous dispersion of the nickel particles, whose average particlesizes in FIG. 1 are approximately 3 μm and in FIG. 2 <10 μm throughout.It can be seen from the picture detail relating to FIG. 1, that fornickel particles having a particle size in the order of magnitude of d≈3μm the average distance D of two particles is about twice that, i.e. D=6μm. This value D likewise is a significant parameter to characterize thematerial.

The table gives experimental values for welding force Fw, erosion E andthe contact resistances Rc during making and breaking. It lists theswitching characteristics of the contacts No. 2 and No. 4, producedaccording to the invention, using as an example the materialcompositions AgNi10 and AgNi40 which are compared with thecharacteristics of conventionally produced contacts No. 1 and No. 3 ofthe same composition.

The electrical test was carried out on convex contacts (r =80 mm) ofdimensions 10 mm×10 mm with 1000 making and breaking operations at AC1000 A, 220 V, cosφ=0.4 and the contact force 60 N. The bounce time ofthe first three jumps was 5 ms with a closing rate of 1.0 m/s and anopening rate of 0.8 m/s at a making angle of 0° and a breaking angle of80°, and a blowout field B=0.5 T/A. The contact resistance test wascarried out at 10 A. Erosion was determined by weighing both contactpieces and forming the average. Based on this, and taking into accountthe theoretical density, the volume erosion was derived.

The table clearly shows that the contact materials No. 2 and No. 4,prepared by methods according to the invention, are distinguished bylower welding force values and by considerably lower erosion rates.

Extensive studies have shown that if mechanically alloyed silver-nickelmaterial is used for switching contacts, a switching microstructure isformed which, compared with conventionally produced materials of thesame composition, is richer in nickel, since in the short duration ofexposure to the arc the finely dispersed nickel can be dissolved in themelt in greater proportion. When the melt cools, this nickelreprecipitates in finely dispersed form.

The melt which, produced from the silver-nickel material according tothe invention, is richer in nickel compared with a previously known AgNimaterial of the same nickel concentration, has a higher viscosity. As aresult, less material is spattered during melting, and contact erosionin the case of the mechanically alloyed material is consequentlyreduced. Furthermore, with the higher-viscosity melt the gas dissolvedin the melt is released in a but lesser proportion, so that duringsolidification of the material pores are formed to a greater extent inthe switching microstructure, which reduce the mechanical strength andthus the welding force.

                                      TABLE                                       __________________________________________________________________________                              Electrical test conditions: 1000 A, 220 V, 1000                               n                                                                        Ni   Fw 99.8%                                               Contact material  grain size                                                                         welding force                                                                        Rcl 99.9%                                                                          Rc3 99.9%                                                                           Erosion                           No.                                                                              composition                                                                           Example    μm!                                                                             N!     mOhm!                                                                              mOhm!                                                                               mm.sup.3 !                       __________________________________________________________________________    1  AgNi 90/10                                                                            Comparative example                                                                     <40  324    0.04 1.69  59.5                              2  AgNi 90/10                                                                            Working example                                                                          <3  257    0.05 2.19  38.0                              3  AgNi 60/40                                                                            Comparative example                                                                     <40  330    0.06 3.10  14.0                              4  AgNi 60/40                                                                            Working example                                                                          <3  194    0.05 1.50   7.7                              __________________________________________________________________________

What is claimed is:
 1. A sintered contact material comprising silver andfrom 5 to 50 weight % nickel, wherein the nickel is in a form of nickelparticles having an average particle size of between 1 μm and 10 μm, andwherein said nickel particles are homogeneously dispersed in amicrostructure of the silver.
 2. The sintered contact material accordingto claim 1, wherein the average particle size of the nickel is less than5 μm.
 3. The sintered contact material according to claim 1, wherein theaverage particle size of the nickel is less than 3 μm.
 4. The sinteredcontact material according to claim 1, wherein the average distancebetween the nickel particles is between 5 and 10 μm.
 5. The sinteredcontact material according to claim 1, wherein the nickel particles areproduced by a griding process.
 6. A method for preparing the sinteredcontact material of claim 1, comprising the steps of introducing nickelparticles into a silver microstructure and subsequently sintering themixture of silver and nickel.
 7. The method according to claim 6,wherein the step of introducing the nickel particles is conducted bymechanical alloying under an air atmosphere.
 8. The method according toclaim 7, wherein either silver powder and nickel powder or a granularmaterial made of silver and nickel is used in the step of mechanicalalloying.
 9. The method according to claim 8, wherein the nickel powderor the granular material used has a particle size distribution of lessthan 500 μm.
 10. The method according to claim 8, wherein the nickelpowder or the granular material used has a particle size distribution ofless than 100 μm.
 11. The method according to claim 8, wherein thenickel powder or the granular material used has a particle sizedistribution of less than 50 μm.
 12. The method according to claim 7,wherein the mechanical alloying is conducted in a ball mill and iscontinued until a lamellar microstructure is formed having nickellamella having a width which is smaller than the particle diameter ofthe nickel starting particles.
 13. The method according to claim 12,wherein the alloying is continued until the nickel lamella have a widthof less than 1 μm.
 14. The method according to claim 7, wherein themechanically alloyed powder is compression-molded and sintered under areductive atmosphere to produce a contact facing.
 15. The methodaccording to claim 14, wherein during sintering nickel lamellae coalesceinto globular particles having a particle size distribution of between 1μm and 10 μm and a particle distance of between 5 and 10 μm.
 16. Themethod according to claim 14, wherein the compression-molding iseffected by extrusion.
 17. The method according to claim 14, wherein thecompression-molding is carried out as a molding technique for contactpieces.
 18. A contact facing produced according to the method accordingto claim 16, wherein said contact facing is fashioned into strips orsections.
 19. A contact facing produced according to the methodaccording to claim 17, wherein said contact facing is fashioned intocontact pieces.
 20. The contact facing according to claim 18, whereinsaid contact facing is formed as a two-layer structure having a firstlayer of silver-nickel and a second layer of pure silver.